Method for producing adiponitrile

ABSTRACT

In a method for producing adiponitrile by electrolysis of an acrylonitrile composite stream passing through an undivided electrolytic cell having a cathode and an anode, flow of the acrylonitrile composite stream to the cell is periodically terminated, a wash stream is passed through the cell in contact with the cathode and anode, said wash stream being a basic aqueous solution having a concentration in the range of about 120 weight percent basic compound in the solution. The wash stream is passed through the cell in contact with the cathode and anode for a period of time in the range of about 1-60 minutes and thereafter the passage of the wash stream through the cell is terminated and the acrylonitrile composite stream is passed to the electrolytic cell for electrolysis.

United States Patent 11 1 Ruehlen Oct. 29, 1974 METHOD FOR PRODUCINGADIPONITRILE [75] Inventor: Forrest N. Ruehlen, Bartlesville,

Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

[22] Filed: July 27, 1972 [21] Appl. No.: 275,556

[56] References Cited UNITED STATES PATENTS 5/1970 Beck et al 204/22210/1971 Havey Primary Examiner--F. C. Edmundson [5 7 ABSTRACT In amethod for producing adiponitrile by electrolysis of an acrylonitrilecomposite stream passing through an undivided electrolytic cell having acathode and an anode, flow of the acrylonitrile composite stream to thecell is periodically terminated, a wash stream is passed through thecell in' contact with the cathode and anode, said wash stream being abasic aqueous solution having a concentration in the range of about 1-20weight percent basic compound in the solution. The wash stream is passedthrough the cell in contact with the cathode and anode for a period oftime in the range of about 1-60 minutes and thereafter the passage ofthe wash stream through the cell is terminated and the acrylonitrilecomposite stream is passed to the electrolytic cell for electrolysis.

1 Claims, 1 Drawing 3,689,382 9/1972 Fox et al 204/73 A SEPARATIONCOLUMN MAKE-UP CELL AQUEOUS BASE SOLVENT ZONE BASE SOLVENT 33 [8 i i a27 RECYCLE Z ADIPONITRILE [ACRYLONITRILE E 217 2 ORGANIC FEED RECYCLE (NBY-PRODUCTS3 ACRYLONITRILE] fwATER+sALTs E 22 MAKE-UP ,WATER SALTS) A a;

METHOD FOR PRODUCING ADIPONITRILE This invention resides in an improvedmethod for producing adiponitrile by electrolysis of an acrylonitrilestream passing through an undivided electrolytic cell having a cathodeand an anode by periodically washing the cathode and anode of the cellwith a basic aqueous wash stream having a concentration in the range ofabout lweight percent basic compound in the solution.

Electrohydrodimerization (EHD). of acrylonitrile to produce adiponitrilehas been found to be economically attractive. Adiponitrile is a valuableprecursor in the manufacture of nylon fibers.

In this process, an aqueous electrolyte, comprising a relatively largeamount of a conducting inorganic salt such as a potassium phosphate, anda small amount of a directing (catalytic) organic salt such astetrabutylammonium phosphate is continuously circulated between twosuitable electrodes under electrolysis conditions. Acrylonitrile iscontinuously added to this circulating stream under conditions whichprovide an emulsion between the two electrodes. As the electrolysisproceeds, a small slipstream is removed from the circulating emulsion.It is allowed to settle, and an organic phase, which contains thedesired adiponitrile product, is drawn off for isolation and recovery ofthe adiponitrile. The aqueous phase is returned to the circulatingemulsion stream. A feature of the abovedescribed process is that itoperates with an undivided cell, that is, there is no membrane orotherform of divider which divides the space between the electrodes into acathode compartment and an anode compartment. Such undivided celloperation has the advantage of simple cell construction and relativelylow internal resistance within the cell.

As is desirable as undivided cell portion is, long term smooth,continuous, and efficient operation is sometimes difficult to achievebecause of the problem of an- 7 odic oxidation of organic feed andproducts. Moreover,

anode corrosion is a frequent problem with undivided cells. For example,when a suitable reaction mixture is circulated between lead electrodes,the conditions under which high conversions and selectivities to desiredproducts are obtained at the lead cathode are generally the sameconditions which provide for significant corrosion of and depositsformation on the lead anode. Moreover, the corrosion of the anode isrelated to the oxidation losses in that it appears to cause even moreoxidation of organic materials which are in the vicinity of thecorroding anode. The products of the corrosion and/or oxidationsometimes tend to increase the internal resistance of the cell therebyrequiring wasteful high levels of cell voltage.

It has now been found that, in a continuous process for theelectro-hydrodimerization of acrylonitrile to adiponitrile wherein asuitable mixture of acrylonitrile, water, and conducting and directingsalts are passed between suitable electrodes in an undividedelectrolytic cell, and wherein an anode is subject to becoming coatedwith insulating deposits, smoother operation including lower voltagerequirements and freedom from severe polarization are obtained byperiodically circulating an aqueous solution of a strong base, such asaqueous potassium hydroxide through the cell to remove the deposits fromthe anode and restore its surface.

This invention, therefore, resides in an improved method for producingadiponitrile by electrolysis of acrylonitrile stream passing through anundivided electrolytic cell having a cathode and an anode byperiodically washing the cathode andanode of the cell with a basicaqueous wash stream having a concentration in the range ofabout 1-20weight percent basic compound in the solution.

Other aspects, objects, and advantages of the present invention willbecome apparent from a study of the disclosure, the appended claims, andthe drawing.

The drawing is a diagrammatic view of the inventive process withapparatus for practicing the method of this invention.

Referring to FIG. 1, acrylonitrile feed and make-up water and salt passinto line 12 by means of lines 10 and 11, respectively. Line 12 carriesa recirculating emulsionstream which passes into and through cellzone 1. The electrodesin the cell are generally positioned such thatthegap between them is vertical. The emulsion can pass through this gapeither from top to bottom or vice versa. Cell zone 1 comprises one ormore cells operating under conditions suitable for the electrolyticconversion of acrylonitrile to adiponitrile. Line 12 carries an aqueoussolution of conducting inorganic salts, such as alkali metal phosphatesalts and a minor amount of a directing tetraalkylammonium (TAA) salt,such as tetrabutylammonium phosphate. Organic products and by-productsare also present. The stream passing through line 12 will hereafter bereferred to as an acrylonitrile composite stream. An emulsified mixtureis removed from cell zone I through line 13, then passes through filter2 andthenpasses into gasdisengager unit 3. In this unit, gases,primarily oxygen, are removed from the system through line 14. Theessentially gasfree liquid stream is then passed from gas disengagerunit 3 into=line 19,:through heater 7, and then back into cell zone 1 bymeans of line .12. The positions of the filter, gas disengager, pump,heater, feed entries, etc., need not necessarily be located in the ordershown in the circulation loop but can be arranged in other sequences.

Cell zone I, for example, operates under conditions which include: atemperature of l50F; an atmospheric or near atmospheric pressure; anemulsion linear velocity of 005-6 ft/sec; an emulsion having an organiccontent of 2-12 weight percent based on the entire liquid stream;suitable electrodes having a composition such aslead, lead alloy,graphite, nickel, mercury, platinum, and the. like; a current density of50-400 amp/ft an emulsion pH of 6-l2; a normality of 0.1-3 N withrespect to inorganic conducting salts; a normality of 0001-03 N withrespect to TAA salts, an acrylonitrile per. pass conversion of up to 10percent; and an overall acrylonitrile conversion of up to 90 percent.

A minor part, corresponding to the desired acrylonitrile conversion, ofthe stream passing through line 19 is diverted and passed through line15 into cooling zone 4 wherein that portion of the emulsion is cooled,using any conventional cooling means to a temperature suitable for theseparation of the emulsioninto separate liquid phases. The chilledemulsion leaves cooling zone 4 through line 16 and passes intoseparation column 5. Any suitable apparatus or combination of apparatusfor settling an emulsified stream into an organic and an inorganic layercan be used in this separation column.

The lower aqueous phase leaves the separation zoneS through line 18 andrejoins the recirculating emulsion stream in line 19. The organic phaseis drawn off from separation column through line 20 and proceeds toseparation zone 6.

Separation zone 6 can and generally will comprise cled water and salts,both organic and inorganic, leave separation zone 6, pass through line24 and also rejoin the recirculating emulsion in stream 19. Therecombined materials in line 19 are blended with fresh acrylonitrilethrough line 10 and make-up water and salt through line ill to-form amixture passing through line 12, through heater 7, wherein the mixtureis adjusted to the desired cell temperature, and thence into cell zone1.

At periodic time intervals, for example, when the cell voltage requiredto maintain a given current flow increases by about 0.5 volt fromnormal, the circulation of emulsion through cell zone 1 is halted, thecell can be drained, and valves 25 and 26 are closed. Valves 27 and 28are then opened and a wash solution contained in zone 8 is recirculatedthrough cell zone 1 by means of pump 29. The wash solution streamcirculates from storage zone 8 through line 30 through cell zone 1 andreturned through line 31 to storage zone 8. The recirculating washingsolution accumulates solid particulate matter from cell zone 1. Thissolid particulate is removed from zone 8, either continuously orintermittently, through line 32. Make-up aqueous base solution is addedto solution storage zone 8 through line 33.

Any suitable cell and electrode configuration which will accommodate acontinuously circulating stream of liquid between the two electrodes canbe used. The cell can be fabricated from any suitable material which iscompatible with the electrolyte, and which is not attacked or corrodedor only slowly corroded, under the operating conditions normallyemployed in the cell.

Preferred materials for the cathode are those having a fairly highhydrogen over-voltage, that is, a hydrogen over-voltage greater thanthat of copper. Examples of such materials include, among others, lead,graphite, nickel, silver, gold, lead alloys, and the like.

Examples of materials suitable for use as the anode include lead, leadalloys, platinum, gold, nickel, iron, and the like.

Lead-containing materials such as metallic lead, lead alloys. or leaddioxide are presently preferred as materials of construction for theanode. Similarly, lead or lead alloys are presently preferred for use informing the cathode.

At periodic intervals, or when the cell voltage has been found toincrease by about 0.5 volt or more over normal, the cell and electrodesare subjected to the washing with aqueous base according to the processof the present invention. Generally speaking, the EHD processisinterrupted at intervals of only 10 to hours, ordinarily, 20-50 hours.

The washing or flushing operation is carried out very simply bytemporarily halting the flow of emulsion through the cell and replacingit with a flow of aqueous base. The circulation of the aqueous base washstream through the cell is carried out for a sufficient time to restorethe surface of the anode to again provide efficient low voltageoperation. Ordinarily, this can be accomplished by circulating theaqueous base for 1-60, preferably 5-30 minutes.

The temperature at which the electrodes are contacted with the aqueousbase can vary widely, ranging from the freezing point to the boilingpoint of the solution. Room temperature operation is generallysatisfactory. With lead dioxide anodes, however, temperatures in excessof F are generally required. The circulation rate can alsovary widely. Alinear velocity through the cell in the range of 005-6 ft/second isdesirable with operation toward the lower end of this range beingpreferred.

The base wash solution which is applicable for use in the inventiveprocess is a strong base which has substantial solubility in water.Thus, preferred bases are alkali metal hydroxides although strongorganic bases such as tetramethylammonium hydroxide can also be used ifdesired. To avoid unnecessarily contaminating the EHD process withforeign cations, the preferred base is one whose cation is alreadypresent'in the process. For example, in a process employing asignificant quantity of potassium phosphates as the circulatingconducting salts, a solution of potassium hydroxide is the preferredaqueous base. The base can be present in the aqueous wash solution inany concentration which is effective for renewing the surface of theanode. Generally, the concentration will be 1-20, preferably 5-15,weight percent basic compound in the solution.

it is presently believed that the benefits of the present invention areobtained by the regular removal of the deposit that forms on the anode.For example, lead is a particularly active materialin the EHD process.However, when using lead electrodes in systems containing salts such asphosphates as well as other organic reactants, a deposit is formed onthe anode which contains combined lead, phosphorus, and organicmaterial. When the anode is contacted with the aqueous base flushingsolution, most, if not all, of the deposit appears to be removed whileit is in a relatively manageable form. The present invention does noteliminate the corrosion of the electrodes such as lead anode corrosion,but it provides management of this type of corrosion so that itinterferes little with the overall EHD process.

Because solid particles corrosion products are thus removed from theanode and passed into the circulating aqueous base solution, it isgenerally desired to provide means by which these particles can beconveniently removed from the process. This can be done easily byfiltering out the solids which accumulate in the solution of aqueousbase or by periodically discharging a settled slurry of solids from theaqueous base storage.

After the suitable solution of aqueous base has been circulated for asufficient time to restore the surfaces of the electrode, the flushingoperation is halted and the circulation of emulsion mixture through thecell and over the electrodes is resumed. if desired, the flushingoperation can be carried out without interrupting the current flow.Thus, some hydrogen will be generated during the flushing operation butthere would be no necessity for the cumbersome off-and-on switching ofhigh flows of current.

EXAMPLE In a process using apparatus and a sequence of steps similar tothat of FIG. 1 acrylonitrile was continuously converted to adiponitrileover a period of 202.7 hours at which time the run was terminatedvoluntarily. The aqueous portion of the recirculating emulsion was asolution of potassium phosphates, being 1.5 N in potassium ion and beingneutralized to a pH of 7.5-7.7 with orthophosphoric acid. At steadystate conditions, this aqueous stream is in the form of an emulsion withacrylonitrile and other organic products. The'total organics amount to4-5 weight percent based on the total emulsion. About 0.1 weight percentof tetrabutylammonium ion was also present in the emulsion.

The system was operated continuously in a manner similar to that of FIG.1 except that neither acrylonitrile nor water nor salts were recycledback into the process from separation zone 6. At 24-36 hour intervals.the voltage across the cell was found to increase from the nominal 4.2volts to a value of about 4.7 volts. At this time. the current and thecirculation of emulsion through cell were halted and a weight percentaqueous solution of potassium hydroxide was circulated through the cellat room temperature for 10-15 minutes. At the completion ofthis flushingperiod, the cell was drained of flushing solution and the current andcirculation of reaction emulsion were resumed through the cell at thenormal operating temperature of l-l 22F.

A summary of other conditions as well as the results of the total runare shown in the following tabulation.

Water 4.23 Acrylonilrilc 19.88 Propionitrile 2.41 Succinonitrile 0.69Tctrabutylammonium ion 0.59 Adiponitrile 67.78 Bis( 2-cyanoethyl )ethcr0. l 7 Hydrotrimer of acrylonitrile l 3.69 Hydrotrimcr of acrylonitrilell 0.28 Methylglutaronitrile 0.24 Unknowns 0.04

The results of this relatively long run show that the cell and electrodeflushing procedure of the present invention permits long periods ofsmooth operation, at a relatively constant and low voltage, free frompolarization or other stoppages, and at high efficiency to desiredproducts.

ln closely identical comparison runs, which were carried out without thebenefit of the invention flushing treatment, the voltage across the cellwas found to slowly increase such that, after several days operation,the voltage required to maintain the desired current density exceededthe capacity of the power supply and the cell became polarized. Thisrequired periodic dismantling of the cell to remove and physically scrubthe anode to restore the effective conducting surface. The presentinvention procedure has obviated the need for such relatively frequentdismantling of the cell.

In addition to EHD runs using lead anodes. equally successful runs weremade under similar conditions using cells which contained a lead cathodeand a lead dioxide anode. At intervals of 22-58 hours, the cell wasflushed with 10 percent KOH solution (by weight) at 180F. The periodicflushing prevented polarization and maintained a relatively low averagevoltage across the cell.

Other modifications and alterations of this invention will becomeapparent to those skilled in the art from Material balance Adiponitrileyield Efficiency to adiponitrile Chemical Current DC Power required(average) Lead Loss Product stream Adiponitrile Acrylonitrile 98.3% 89.5lb/lOO lb acrylonitrile converted 87.7% (based on acrylonitrile) 86.5%

l.l5 KWH/lb adiponitrile 0.0067 lb/lb adiponitrile 0.664 wt. fraction0.2l0 wt. fraction A typical organic ph ase effluent from the ra'c'ss'withe composition of this product accumulated for the final 24-hour periodand is shown below in weight percent.

the foregoing discussion and""Ec'tfipari'yifiitfiawi'hg;"' and it shouldbe understood that this invention is not to be unduly limited thereto.

What is claimed is:

1. In a method for producing adiponitrile by electrolysis of anacrylonitrile composite stream passing through an undivided electrolyticcell having a cathode and an anode, the improvement comprising:

periodically terminating flow of the acrylonitrile composite stream tothe cell; passing a wash stream through the cell in contact with thecathode and the anode, said wash stream 'being a strong base aqueoussolution having a concentration in the range of about l weight percentbase compound in the solution;

continuing to pass the wash stream through the cell in contact with theelectrodes'for a period of time sufficient for removing deposits fromthe electrodes; I

terminating the passage of the wash stream through the cell; and

initiating passage of the acrylonitrile composite stream through thecell and the production of adiponitrile.

2. A method, as set forth in claim 1, wherein the acrylonitrile streamis terminated and the wash stream is passed through the cell after theacrylonitrile stream has passed operably through the cell for a periodof time in the range of about 10-100 hours.

3. A method, as set forth in claim 1, wherein the acrylonitrile streamis terminated and the wash stream is passed through the cell after thepower requirements for the electrolysis process have increased to avalue greater than about 0.5 volt.

4. A method, as set forth in claim 1, wherein the wash stream is passedthrough the cell at a linear velocity in the range of about 0.05 toabout 6.0 feet per second.

5. A method, as set forth in claim 1, wherein at least one of thecathode or anode is formed of lead dioxide and including maintaining thewash stream at a temperature greater than about 150F.

6. A method, as set forth in claim 1, wherein the wash stream isfiltered after passing through the cell.

7. A method, as set forth in claim 1, wherein the wash stream comprisestetramethylammonium hydroxide.

8. A method, as set forth in claim 1, wherein cations of the wash streamare of the same type as cations of material discharging from the cellduring electrolysis of the adiponitrile stream.

9. A method, as set forth in claim 1, wherein the flow of theacrylonitrile composite stream to the cell is terminated in response toone of the power requirements of the electrolytic process havingincreased to a value greater than about 0.5 volt or the operation of theelectrolytic cell for a period of time in the range of about 10-100hours, the basic compound of the wash stream is at a concentration inthe range of about l-20 weight percent, and the time for passing saidwash stream through the cell is a period of time in the range of aboutl-60 minutes. I

10. A method, as set forth in claim 9, wherein the wash stream ismaintained at a temperature greater than about F.

11. A method, as set forth in claim 10, wherein the wash streamcomprises cations of the same type as cations of material dischargingfrom the cell during electrolysis of the composite stream.

12. A method, as set forth in claim 9, wherein the wash stream comprisescations of the same type as cations of material discharging from thecell during electrolysis of the composite stream.

13. A method as set forth in claim 1, wherein the strong base is analkali metal hydroxide.

14. In the method for producing adiponitrile by electrolysis of anacrylonitrile composite stream passing through an undivided electrolyticcell having a cathode and an anode, the improvement comprising:

terminating the flow of the acrylonitrile composite stream to the cellin response to the cell voltage increasing at least 0.5 volt during theoperation of said cell;

passing a potassium hydroxide solution wash stream through the cell incontact with the electrodes for a period of time in the range of about1-60 minutes, said wash stream having a potassium hydroxideconcentration in the range of about 1 to about 20 weight percent, saidwash stream also comprising cations of the same type as cations ofmaterial discharging from the cell during electrolysis of the compositestream, and said wash stream being maintained at a temperature greaterthan about 150 F.;

terminating the passage of the wash stream through the cell; and

initiating passage of acrylonitrile composite stream through the celland the production of adiponitrile.

1. IN A METHOD FOR PRODUCING ADIPONITRILE BY ELECTROLYSIS OF ANACRYLONITRILE COMPOSITE STREAM PASSING THROUGH AN UNDIVIDED ELECTROLYTICCELL HAVING A CATHODE AND AN ANODE, THE IMPROVEMENT COMPRISING:PERIODICALLY TERMINATING FLOW OF THE ACRYLONITRILE COMPOSITE STREAM OFTHE CELL; PASSING A WASH STREAM THROUGH THE CELL IN CONTACT WITH THECATHODE AND ANODE, SAID WASH STREAM BEING A STRONG BASE AQUEOUS SOLUTIONHAVING A CONCENTRATION IN THE RANGE OF ABOUT 1-20 WEIGHT PERCENT BASECOMPOUND IN THE SOLUTION; CONTINUING TO PASS THE WASH STREAM THROUGH THECELL IN CONTACT WITH THE ELECTRODES FOR A PERIOD OF TIME SUFFICIENT FORREMOVING DEPOSITS FROM THE ELECTRODES; TERMINATING THE PASSAGE OF THEWASH STREAM THROUGH THE CELL; AND INTIATING PASSAGE OF THE ACRYLONITRILECOMPOSITE STREAM THROUGH THE CELL AND THE PRODUCTION OF ADIPONITRILE. 2.A method, as set forth in claim 1, wherein the acrylonitrile stream isterminated and the wash stream is passed through the cell after theacrylonitrile stream has passed operably through the cell for a periodof time in the range of about 10-100 hours.
 3. A method, as set forth inclaim 1, wherein the acrylonitrile stream is terminated and the washstream is passed through the cell after the power requirements for theelectrolysis process have increased to a value greater than about 0.5volt.
 4. A method, as set forth in claim 1, wherein the wash stream ispassed through the cell at a linear velocity in the range of about 0.05to about 6.0 feet per second.
 5. A method, as set forth in claim 1,wherein at least one of the cathode or anode is formed of lead dioxideand including maintaining the wash stream at a temperature greater thanabout 150*F.
 6. A method, as set forth in claim 1, wherein the washstream is filtered after passing through the cell.
 7. A method, as setforth in claim 1, wherein the wash stream comprises tetramethylammoniumhydroxide.
 8. A method, as set forth in claim 1, wherein cations of thewash stream are of the same type as cations of material discharging fromthe cell during electrolysis of the adiponitrile stream.
 9. A method, asset forth in claim 1, wherein the flow of the acrylonitrile compositestream to the cell is terminated in response to one of the powerrequirements of the electrolytic process having increased to a valuegreater than about 0.5 volt or the operation of the electrolytic cellfor a period of time in the range of about 10-100 hours, the basiccompound of the wash stream is at a concentration in the range of about1-20 weight percent, and the time for passing said wash stream throughthe cell is a period of time in the range of about 1-60 minutes.
 10. Amethod, as set forth in claim 9, wherein the wash stream is maintainedat a temperature greater than about 150*F.
 11. A method, as set forth inclaim 10, wherein the wash stream comprises cations of the same type ascations of material discharging from the cell during electrolysis of thecomposite stream.
 12. A method, as set forth in claim 9, wherein thewash stream comprises cations of the same type as cations of materialdischarging from the cell during electrolysis of the composite stream.13. A method as set forth in claim 1, wherein the strong base is analkali metal hydroxide.
 14. In the method for producing adiponitrile byelectrolysis of an acrylonitrile composite stream passing through anundivided electrolytic cell having a cathode and an anode, theimprovement comprising: terminating the flow of the acrylonitrilecomposite stream to the cell in response to the cell voltage increasingat least 0.5 volt during the operation of said cell; passing a potassiumhydroxidE solution wash stream through the cell in contact with theelectrodes for a period of time in the range of about 1-60 minutes, saidwash stream having a potassium hydroxide concentration in the range ofabout 1 to about 20 weight percent, said wash stream also comprisingcations of the same type as cations of material discharging from thecell during electrolysis of the composite stream, and said wash streambeing maintained at a temperature greater than about 150* F.;terminating the passage of the wash stream through the cell; andinitiating passage of acrylonitrile composite stream through the celland the production of adiponitrile.